1. Field of the Invention
The present invention relates to an automatic gain control apparatus for a video signal and an adjusting method thereof, and more particularly to an automatic gain control apparatus which does not need initial AGC (Automatic Gain Control) adjustment and an adjusting method thereof.
The present application is based on Korean Patent Application No. 2002-0039458, filed Jul. 8, 2002, which is incorporated herein by reference in its entirety.
2. Description of the Related Art
A TV receiver automatically controls a gain of a receiver according to intensity of a received radio wave, and has an AGC (Automatic Gain Control) circuit for obtaining a constant video signal detecting output without distortion.
FIG. 1 is a conventional block diagram of an automatic gain control circuit for a TV receiver.
As shown in FIG. 1, the conventional automatic gain control circuit (AGC circuit) for a TV receiver comprises a tuner 12, a pre-filter 13, a SAW (Surface Acoustic Wave) filter 14, an IF(Intermediate Frequency) amplifier 15, a demodulator 16, an IF AGC 17 and a RF (Radio Frequency) AGC 18.
High frequency (RF) signal from a predetermined channel selected by the tuner 12, is input through an antenna 11. An RF amplifier 12b amplifies the input high frequency signal, converting the same into an intermediate frequency (IF) signal.
The IF signal passes through the pre-filter 13 and the SAW filter 14, and is amplified at the IF amplifier 15.
The amplified IF signal is demodulated at the demodulator 16, and is output in the form of a composite video signal.
A RF AGC 18 and an IF AGC 17 detect the composite video signal demodulated at and provided from the demodulator 16, controlling the RF amplifier 12b and the IF amplifier 15 for obtaining a predetermined gain.
In the case that the intensity of an input signal is strong, i.e., if it is over a predetermined degree, the RF AGC 18 controls an amplifying gain at a RF terminal, while controlling to obtain a constant gain at an IF terminal without suppressing the gain.
In the case that the intensity of an input signal is weak, i.e., if it is below a predetermined degree, the IF AGC 17 controls the amplifying gain at the IF terminal, while controlling to obtain a constant gain at the RF terminal without amplifying the gain.
In a conventional automatic gain control circuit for a TV receiver, the RF AGC 18 and the IF AGC 17 detect a demodulated composite video signal, and control the RF amplifier 12b and the IF amplifier 15, respectively, to maintain a constant voltage gain.
FIGS. 2A and 2B are graphs representing a RF AGC curve for the conventional automatic gain control circuit of a TV receiver as shown in FIG. 1.
As shown in FIGS. 2A and 2B, if a level A of a RF input level is high, an AGC voltage B fit for the relevant level in the AGC curve, is fed back to the IF AGC 17 and the RF AGC 18.
An AGC constant determines an operation point of the AGC graph. In the case that the AGC constant becomes large, the AGC graph moves to the right, showing a saturation phenomenon of a signal, while in the case that the AGC constant becomes small, the AGC graph moves to the left, causing a noise in a signal.
The following mathematical equations 1 define the AGC curve.B=−G*A  [Equation 1]where, B is an AGC voltage, G is an AGC constant, A is a CVBS(Composite Video Baseband Signal) signal level,O=(K+B)*I  [Equation 2]where, O is an IF/RF amplifier output, K is an amplifying rate of amplifier, and I is an input signal of amplifier.
Further, in the case that an amplifying characteristic deteriorates due to an error during part manufacturing process of the RF amplifier or other reasons, and a signal is insufficiently amplified, a weak signal passes through the pre-filter 13, the SAW filter 14, the IF amplifier 15, the demodulator 16, whereby a weak composite signal is output.
When detecting the weak composite signal, the AGC circuit raises an amplifying rate of the IF amplifier, instead of the insufficiently amplifying RF amplifier, to form 1 Vpp signal.
In the case that an amplifying characteristic deteriorates due to an error during part manufacturing process of the RF amplifier or other reasons, and a signal is excessively amplified, the AGC circuit lowers an amplifying rate of the IF amplifier, forming 1 Vpp signal.
Under such circumstance, a weak signal vulnerable to noise passes through the pre-filter 13, the SAW filter 14, the IF filter 15, whereby the noise characteristic of the signal deteriorates or the signal is excessively amplified, causing a saturation phenomenon.
Therefore, in the case that an amplifying characteristic deteriorates due to an error during part manufacturing process of the RF amplifier or other reasons, and a signal is insufficiently amplified, an automatic gain control circuit for adjusting a control voltage of the RF AGC, is required.